Magnetoresistive sensors can include anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR), tunnel magnetoresistive (TMR) and other technologies, referred to collectively as XMR technologies. XMR sensors can be used for a variety of applications, including magnetic field and current sensors, speed sensors, rotation sensors, angle sensors, among others.
GMR and TMR sensors are based on magnetoresistive elements having a free layer, which aligns its magnetization with an external magnetic field, and a reference layer, which is not affected by the external magnetic field. The magnetization of the free layer rotates with respect to the reference layer, resulting in a change of the resistance of the magnetoresistive elements.
An angle sensor comprises two types of full bridges—a sine bridge and a cosine bridge—having reference magnetizations that are perpendicular with respect to one another. From the sine and cosine of these bridges an arc tangent can be calculated to obtain an angle of the external magnetic field. An AMR angle sensor has an angle uniqueness of 180 degrees due to the 90-degree uniqueness of the AMR effect with respect to a rotating external in-plane field, and GMR and TMR angle sensors have an angle uniqueness of 360 degrees. The GMR and TMR angle sensors, however, suffer from accuracy drift, especially at high magnetic fields in combination with high temperatures. AMR angle sensors, as opposed to GMR and TMR angle sensors, are therefore generally used in applications with high accuracy requirements.
An angle sensor, as opposed to a speed sensor, does not measure an external magnetic field's magnitude. An angle sensor's sine and cosine bridges output sine and cosine signals, respectively, for determining the external magnetic field's direction only. The angle sensor works in saturation mode; the magnetization of the free layer is saturated and follows the external magnetic field, but there is no information about the strength of the external magnetic field. A speed sensor measures the strength of the magnetic field and the sine. An angle sensor measures the direction of the external magnetic field, and the sine and cosine.
There is a need for an angle sensor with improved accuracy, especially for automotive safety applications. To achieve corresponding targets in terms of Automotive Safety Integrity Level (ASIL), different target parameters such as Failure In Time (FIT) rate, diagnostic coverage, Single Point Fault Metric (SPFM), Latent Point Fault Metric (LPFM), etc. have to achieve a predetermined value. For angle sensors, a typical safety goal guarantees an angle accuracy within a predefined time, for example, 5° deviation of the actual angle value detected in 5 ms. A problem to be overcome in guaranteeing the validity of the sensed angle value.
Diversity is an important element used to ensure a high functional safety level. Diversity in this context means that information is provided by different elements or technologies. This disclosure is directed to achieving diversity, by means other than merely doubling of sensor elements.